[中文摘要 ] 立霧溪流域面積約佔太魯閣國家公園的三分之二，不但孕育著名的「太魯閣峽谷」，其河道形態與水沙特性也構成 溪流生態的環境基礎，具有地形、地質與生態上的獨特性與重要性。本溪所蘊藏的水力資源自日據時期以來就被規劃為 台灣東部電力開發的目標區，歷年來所興修的建物主要包括立霧電廠、溪畔壩、砂卡礑壩，以及相關道路系統。而立霧 電廠的營運與人工壩堤和道路的存在對立霧溪河道特性的直接、間接影響，經常是電力開發與環境保育議題的焦點。 為進一步探究此問題，本研究從流域地形系統的觀點出發，以河道地形次系統為焦點，建立本溪的「河道行為與形 態特徵」，並以定期觀測資料為基礎，探討「溪畔壩對河道形態之影響」。研究結果顯示，立霧溪屬山地型底岩河道， 與邊坡崩塌關係密切的巨礫灘和河流寬度、坡度是決定天祥以下整體河道形態的主因，而豪雨期的洪峰流量是局部改變 河床（表層）沈積物特性與河道橫剖面形態的主要有效營力。 根據近一年來的實察資料，溪畔壩對立霧溪河道的影響至少包括三個方面。(1) 全年度截取河水，使溪畔以下主流流 量減少一半以上，進而降低本溪搬運沈積物的能力，河流負荷物滯留河道中的時間被延長。(2) 溪畔壩以上蓄水區因壩體 攔截使水位提高，流速減緩而使泥沙顯著淤積，影響範圍大致到布洛灣野溪匯入點之下游側（回水之上限）。(3) 在洪峰 減退時為調節水位、掌握蓄水量而進行的排洪、排沙作業，可使鄰近壩體下游側的河道形態沖淤變化量達6公尺，尤其以 白沙橋以上至溪畔壩間最為顯著。此外，此種排洪、排沙的調節作業也增加高懸移質水流（混濁水流）通過下游河道的 時間，影響景觀。 [Abstract] The Gorgeous Gorge, the Marvelous Marble! Located on the eastern flank of the Central Range, the Liwu River exhibits as a typical mountainous bedrock channel. Within the environmental context of rapid uplift rate and high denudational processes, it not just creates the worldwide famous Taroko Gorge but is also regarded as one of the major rivers with abundant potential hydropower in eastern Taiwan. The conflict between exploitation of the hydropower resource and natural conservation has become intensified since the Taroko National Park was founded in 1986. The aim of this project is, thus, to examine the impact of engineering structures, Hsipan Dam in particular, to the channel morphology and natural landscape of the Liwu River. A comprehensive investigation and synthesis of all available archival information and environmental datasets has been conducted and, for the first time, the fluvial system of the Liwu River has been characterized with the channel sub-system as the central theme. It is also revealed that channel width, channel slope and the location of river bars consisting of extremely-large boulders (which are mainly from adjacent valley slopes) are major factors of channel morphology of the Taroko gorge. The cross section of the channel has also been surveyed at several sites between Pulowan and Eternal Spring Shrine with a laser ranger over a year long period. The result shows the operation of the Hsipan Dam has at least three direct geomorphic impacts. These are: (1) downstream discharge has been significantly decreased due to the very frequent water diversion, which in turn reduces the river capacity to move sediments; (2) the interception of the dam has caused mean water level rising on its upstream side and this also results in obvious silt-up in the backwater zone; (3) the practice of releasing floodwater and/or sediments, especially during the falling period of the peak flood, may cause the morphological change of cross-section up to 6 m in the immediate downstream stretch of the dam.